I was the one that posted that. I had some customer 5/8 bearing spindles machined to fit over the ends of my MBS mountainboard to handle the larger 5/8 bearing diameter wheels you can buy at Home Depot for hand trucks. They are larger and wider, so the footprint is better for sand since the weight is distributed more evenly to the ground.
I had them custom made off some CAD drawings I made up. Using a setscrew to hold them in place and a nylon nut to hold the tires, just like the original nylon nuts.
They fit over the existing axle on each side, are about 5.5 inches long, and taper down to 5/8" for I think (from memory) 3" to allow the new tire to fit on.
It works really well. Give me a couple days and I'll put the graphic on the site. Let me know if you want me to do it by emailing me at the site.